The title compound, [Cu4(C2O4)2(N3)4(C5H9N3)2]·0.9H2O, contains a tetranuclear CuII-based molecule composed of two oxalate-bridged CuII dimers linked through end-on azide ions and related by an inversion center. The tetranuclear unit contains two crystallographically independent CuII ions. One CuII ion coordinates to two N atoms of a histamine molecule, two O atoms of a bridging oxalate ligand, and an N atom of an end-on bridging azide ligand, leading to an elongated square-pyramidal coordination geometry in which the azide ion occupies the axial position. The other CuII ion, which has a square-planar coordination geometry, is coordinated by two O atoms of a bridging oxalate ligand and two N atoms of two different azide ligands, one which is bridging. In the crystal, a two-dimensional network parallel to (010) is formed by N-HN and N-HO hydrogen bonds. A partially occupied solvent water molecule refined to an occupancy of 0.447 (5). Two of the azide ligands were refined as disordered over two sets of sites with refined occupancies in the ratios 0.517 (8):0.483 (8) and 0.553 (5):0.447 (5).

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: LH5597 ).

Acknowledgements

CL wishes to thank the Vice President (Grenfell Campus) Research Fund 208384 47333 2000 from Grenfell Campus, Memorial University of Newfoundland, for supporting this work. KAA wishes to acknowledge the National Science Foundation and the University of Florida for funding the purchase of the X-ray equipment.

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